Hold the mangoes standing and with a knife, cut from the top down on one side of the pit to separate flesh. Repeat on other side. Using a spoon, scoop flesh and coarsely chop. Scrape flesh from pits, if any. Discard pits.
In a pot, combine mango, sugar, and lemon juice. Bring to a boil until sugar is dissolved.
Lower heat and cook, mashing with the back of spoon, for about 25 to 35 minutes or until mixture thickens.
To check for doneness, use the "parting the sea" technique by transferring a teaspoonful onto a small dish and refrigerate for about 1 minute. Run your finger in the middle of jam. If it remains open and does not flow back together, it is ready.
Transfer mango jam to jars while hot. Allow to cool before sealing completely and store in refrigerator.